.back_top {
    width: 28px;
    height: 46px;
    background: url("../images/back_top.png") no-repeat 0 0;
    background-size: 66px 46px;
    display: block;
    position: fixed;
    left: 50%;
    margin-left: 630px;
    bottom: 88px;
}

.back_top:hover {
    background-position: -38px 0;
}
